1. Field of the Invention
This invention pertains to the field of communication networks and, more particularly, to a system and method of communication in a burst-type communication network.
2. Background of the Related Art
In a burst-type communication network, it is inefficient to dedicate communication channels to individual communication terminals which may only transmit sporadically. Consequently, in many burst-type communication networks, communication terminals share communication resources.
Typically, a burst-type communication terminal transmits data as a series of data packets, containing a packet header and a data payload. The packet header is often used by a receiving terminal to detect and synchronize the receiver to the data transmission. The header also may contain information regarding the data transmission, such as packet length, data format, transmitter ID, receiver ID, etc.
Burst-type communication networks employ a variety of protocols for sharing limited communication resources among multiple communication terminals. One well-known protocol used in communication networks which transmit packetized data is the Aloha (also called “pure Aloha”) communication protocol.
In a network using the pure Aloha protocol, any communication terminal in the network may initiate a data transmission at any random time within a time frame. Because of the random times at which a terminal may initiate a data transmission, two or more terminals may initiate a data transmission at overlapping times, resulting in a “collision.”
Transmissions involved in such collisions arrive at a receiver with errors. After a suitable delay without receiving acknowledgments confirming successful reception, the transmitters retry the transmissions. Of course these transmissions may also encounter collisions and therefore may also be unsuccessful. The terminals continue transmitting with suitable delay between transmissions, until the transmissions are received without error and acknowledged. Collisions reduce the throughput efficiency of the network. Maximum throughput efficiency in a pure Aloha network is 18.4%.
One important variation of the Aloha protocol is called “slotted Aloha”. FIG. 1 illustrates principles of a slotted Aloha communication protocol. As shown in FIG. 1, a communication network using a slotted Aloha communication protocol divides time into a series of time slots 124, which are usually organized into a repetitive series of longer time periods called “time frames”. All data transmissions from any communication terminal in the communication network must begin and end within a time slot. If a communication terminal has a data transmission which is longer than a time slot period, then it must break the data transmission up into two or more shorter data transmissions which each fit within a time slot period.
Nevertheless, in a slotted Aloha network, any communication terminal may transmit in any slot and so collisions still occur. Maximum throughput efficiency in a slotted Aloha network is 36.8%.
To reduce collisions, some communication networks employ a slotted Aloha protocol which includes a reservation feature, assigning a portion of the time slots in each time frame as reserved for exclusive use by designated terminals. In a reservation protocol, a communication terminal having a message to transmit may first transmit a special message called a reservation request. A network hub or controller monitors the reservation request and assigns one or more reserved time slots to the requesting terminal during the following time frame or frames. The hub broadcasts the reservation to the communication network so that all other terminals in the network avoid transmitting during the reserved time slot(s).
FIG. 1 shows reserved time slots 130 in each time frame 110. The use of reservations with a slotted Aloha protocol increases overall throughput efficiency for the network.
In general, reservation protocols may use two types of reserved capacity:
temporary and indefinite. Temporary reserved capacity allocates reserved time slots on a time-slot-by-time-slot basis within a frame. Indefinite reserved capacity is allocated as reserved time slots which are set aside for use by designated transmitters for an indefinite number of consecutive time frames.
Typically, a communication network may transmit data packets with a wide variety of packet lengths. High network throughput efficiency requires the use of time slots with lengths which are well matched to the data packets to be transmitted. If a data transmission is shorter than a time slot, part of the slot period will be wasted, reducing efficiency. If a data transmission is too long for a time slot, the data transmission will have to be broken into multiple time slots. In that case, some of the packet header information and other overhead for the data transmission is repeated in each time slot, again reducing efficiency.
To improve efficiency in a slotted Aloha network, all time slots may not have equal length. Short data packets can be transmitted in short time slots and long data packets can be transmitted in long time slots. FIG. 1 shows time frames in a slotted Aloha network with reservations where the reserved time slots have unequal lengths.
However, the problem remains to match the mix of time slots of different lengths to the mix of packets being transmitted. Also, for many communication networks, the mix of data transmission lengths varies with time, e.g., packets transmitted during the day may have different lengths than those transmitted at night.
Accordingly, it would be advantageous to provide a communication method and network which operates with various changing mixtures of packet lengths with greater throughput efficiency than conventional slotted Aloha networks. It would also be advantageous to provide a communication method and network which automatically adjusts the allocation of reserved time slots when traffic loads are high to increase efficiency. Other and further objects and advantages will appear hereinafter.